ETIOLOGY

Corynebacterium pseudotuberculosis causes the classical disease. It is a soil-borne organism that gains access to tissue through wounds or insect bites. C. pseudotuberculosis possesses a cytotoxic surface lipid coat that appears to facilitate intracellular survival and abscess formation. It also produces a phospholipase exotoxin that increases vascular permeability, has an inhibitory effect on phagocytes and may facilitate spread of infection in the host. Biotypes associated with ulcerative lymphangitis in cattle and horses are different from the biotype associated with caseous lymphadenitis in sheep and goats.

EPIDEMIOLOGY

PATHOGENESIS

Infection of skin wounds is followed by invasion of lymphatic vessels and the development of abscesses along their course. Generalized lymph node involvement is unusual. The organism possesses a cytotoxic surface lipid coat that appears to facilitate intracellular survival and abscess formation and produces a phospholipase exotoxin that increases vascular permeability and has an inhibitory effect on phagocytes.

CLINICAL FINDINGS

CLINICAL PATHOLOGY

The isolation of C. pseudotuberculosis from discharging lesions is necessary to confirm the diagnosis. There are no serological tests validated for infection in cattle or horses but serological tests used for serological diagnosis of caseous lymphadenitis in sheep are used for diagnosis of C. pseudotuberculosis in horses in some laboratories.

TREATMENT

C. pseudotuberculosis is sensitive to all common antimicrobial drugs with the exception of the aminoglycoside group, but systemic treatment of infected animals does not affect the recovery period.⁴ Local treatment of ulcers is the usual and most effective procedure but parenteral injections of penicillin or tetracycline may be necessary in severe cases. In the early stages an autogenous bacterin may have value as treatment.

CONTROL

Good hygiene in stables and under-foot, careful disinfection of injuries to the lower limbs usually afford adequate protection against ulcerative lymphangitis. With all manifestations, isolation of the affected animals and local treatment with antiseptics act to prevent the disease from spreading to other animals in the herd. Infected animals should not be introduced to a naive herd and there should be adequate fly control.

ETIOLOGY

Corynebacterium pseudotuberculosis is the specific cause of this disease.

EPIDEMIOLOGY

The disease is spread from animal to animal by means of contaminated grooming utensils or harness. An existing seborrhea or folliculitis due to blockage of sebaceous gland ducts by pressure from harness probably predisposes to infection. Inefficient grooming may also be a contributing cause.

Contagious acne is of limited occurrence and causes temporary inconvenience when affected horses are unable to work.

PATHOGENESIS

Infection of the hair follicle leads to local suppuration and the formation of pustules which rupture and contaminate surrounding skin areas. Occasional lesions penetrate deeply and develop into indolent ulcers.

CLINICAL FINDINGS

The skin lesions usually develop in groups in areas which come into contact with harness. The lesions take the form of papules which develop into pustules varying in diameter from 1–2.5 cm. There is no pruritus but the lesions may be painful to touch. Rupture of the pustules leads to crust formation over an accumulation of greenish-tinged pus. Healing of lesions occurs in about 1 week but the disease may persist for 4 or more weeks if successive crops of lesions develop.

CLINICAL PATHOLOGY

Swabs of the lesions can be taken to determine the presence of C. pseudotuberculosis.

TREATMENT

Affected animals should be rested until all lesions are healed. Frequent washing with a mild skin disinfectant solution followed by the application of antibacterial ointments to the lesions should facilitate healing and prevent the development of further lesions. Parenteral administration of antibiotics may be advisable in severe cases.

CONTROL

Infected horses should be rigidly isolated and all grooming equipment, harness and blankets must be disinfected. Grooming tools must be disinfected before each use. Vaccination is not likely to be effective because of the poor antigenicity of the organism.

ETIOLOGY

Rhodococcus equi is a Gram-positive, pleomorphic rod. The most important manifestation of R. equi infection is pneumonia in foals. It also causes pleuropneumonia, pneumonia, osteomyelitis, and abortion in immunocompromised and normal adult horses, abscesses that must be differentiated from tuberculosis in pigs and ruminants, and pneumonia in immunosuppressed humans.^1-5 The organism is a natural inhabitant of soil, grows well at temperatures ranging from 10–40°C, and is readily isolated from the feces of herbivores and their environment. However, isolates of R. equi vary in virulence, with many isolates obtained from feces or soil not being pathogenic.⁶

There are a large number of virulent strains of R. equi, based on pulsed-field gel electrophoresis of chromosomal DNA.^7,8 While there is evidence of clustering of strains on farms and on most farms 1 or 2 strains predominate, there is little evidence of marked regional variations in prevalence of strains of virulent R. equi.^7,8 Only rarely will it be possible to link infections to a given site or region on the basis of analysis of chromosomal DNA.⁷

Virulence of R. equi is dependent upon the ability of the organism to enter, survive in and replicate in macrophages. Virulence is associated with the presence of highly immunogenic virulence associated proteins (Vap A, C, D, E, G, H), of which Vap A is apparently the most important, although the role of the other Vap proteins has not been determined.^9,10 Vap A is a surface-expressed, lipid-modified protein that elicits an intense humoral response by foals.¹⁰ Expression of Vap A, C, D, and E is upregulated by incubation at 37°C, consistent with their role as virulence factors. Other genes probably involved in virulence are also upregulated by conditions that mimic those in vivo.¹¹ The presence of the virulence proteins is associated with enhanced ability of virulent R. equi to survive and replicate within macrophages, whereas avirulent strains replicate poorly or not at all.¹² However, the exact virulence mechanisms of R. equi are not known.¹³

The genes for Vaps A and C–H are present in a pathogenicity island in an 80–85 kb plasmid that is present in 98% of isolates of R. equi from foals with pneumonia.¹⁴ Most isolates from the environment, feces, pigs, cattle, and human patients with R. equi infection do not contain either of the two identified virulence plasmids.^4,15,16 Virulence is associated with the presence of the plasmid, and loss of the plasmid by a strain of R. equi results in loss of virulence. There is geographical variation both within and between countries in the prevalence of each of the virulence plasmids in isolates from foals with pneumonia.^15,17,18

EPIDEMIOLOGY

PATHOGENESIS

Exposure of foals to R. equi is common, based on rate of seroconversion, yet the development of respiratory infection and clinical disease is much less common. The reason for this is not fully understood, although development of the disease probably depends on exposure to an infectious dose of organism and the susceptibility of the foal. Recent epidemiology studies suggest that exposure occurs within the first few days of life, before waning of maternally derived passive immunity, and that affected foals have lower CD4⁺:CD8⁺ ratios before development of the clinically apparent disease than foals that do not subsequently develop the disease.³⁶ The differences between groups of foals was largest during the first 2 weeks of life, suggesting that foals that subsequently develop disease associated with R. equi may have impaired immune function during early life.³⁶ In adult horses, in which the disease is rare, protective immunity is associated with both cellular and humoral immune responses characterized by enhanced immunoproliferative responses of CD4⁺ and CD8⁺ cells and presence of IgGa and IgGb antibodies to Vap A.³⁷ Opsonizing antibody to R. equi is an important defense mechanism in experimentally infected foals and administration of R. equi hyperimmune plasma or plasma rich in anti-Vap A and C antibodies protects experimentally infected foals from developing pneumonia.³⁸ Overall, these results suggest that foals that develop R. equi pneumonia have a T helper cell (Th)2-like response to infection, rather than a Th1-like response. Th1-like responses, which are associated with enhanced CD4⁺ and CD8⁺ responses, are believed to be important in resistance to the disease.¹³ Whether the switch to a Th2-like response to infection is a function of virulent R. equi or an attribute of susceptible foals has not been determined.

Experimental and clinical studies indicate that the foal is infected several weeks or months before clinical signs are observed. Virulent strains of R. equi are facultative intracellular parasites of macrophages, which they ultimately destroy. Neutrophils are bactericidal for R. equi but the organism can survive by inclusion in macrophages. Opsonization of R. equi by specific antibodies results in enhanced lysosome–phagosome fusion and greater killing of R. equi by equine macrophages and monocytes,¹³ whereas entry of R. equi into macrophages by nonimmune phagocytosis is not associated with enhanced killing. Its survival in the macrophage is associated with absence of phagosome– lysosome fusion.³⁹ Nonvirulent strains do not proliferate in macrophages and monocytes. The combined action of humoral and cellular immune systems is important in preventing development of the disease after inhalation of bacteria. Without opsonization, the capacity of the pulmonary macrophage of foals to kill R. equi is impaired and the organism has been shown to be able to persist in the pulmonary macrophage of infected foals. The inability of the pulmonary macrophages to destroy R. equi leads to persistent infection in the lung and a chronic bronchopneumonia with extensive abscessation and an associated suppurative lymphadenitis.

Intestinal infection is common in foals with R. equi pneumonia,⁴⁰ although clinical manifestations of the intestinal infection, such as diarrhea, are uncommon. Gastrointestinal tract infection is characterized by ulcerative lesions of the mucosa of the large intestine and cecum. In rare cases bacteremia and subsequent suppurative foci may develop in many organs, including bones and joints, liver, kidneys, and subcutis.

CLINICAL FINDINGS

R. equi pneumonia of foals may present as an acute onset of inappetence, fever, depression and tachypnea or as a more chronic disease characterized by cough and failure to thrive. The former presentation is more common and usually occurs in foals less than 3–4 months of age, with younger foals being more severely affected. It is important to realize that the acute onset of the disease is preceded by a long incubation period during which clinical signs are minimal and that the development of clinical signs is associated with severe and extensive lung lesions. The foal is often in respiratory distress and is reluctant to move and to suckle. Cyanosis may be present in severe cases. Auscultation of the chest may reveal crackles and wheezes, but abnormal lung sounds are often much less apparent than the severity of the respiratory disease suggests they should be. Foals with R. equi abscesses may not have abnormal lung sounds. There is usually minimal nasal discharge.

Subcutaneous abscesses, osteomyelitis and septic arthritis may be present or develop.⁴¹ Many foals (20%) have an aseptic polyarthritis, evident as nonpainful distension of joints, at the time they develop signs of respiratory disease. Immune-mediated uveitis occurs in approximately 10% of severely affected foals.⁴²

In older foals the disease assumes a characteristic clinical syndrome marked by development of severe lesions without clinical signs in the foal. A subacute pneumonia develops slowly with coughing, an increase in the depth of respiration with respiratory distress developing in the late stages, and characteristic loud, moist crackles or ‘rattles’ on auscultation. The foal continues to suck and the temperature is sometimes normal but the foal becomes emaciated. Severe diarrhea may follow or accompany the respiratory signs. Nasal discharge and lymph node enlargement in the throat regions are absent. Severely affected animals die in 1–2 weeks. Thoracic auscultation may not reveal any abnormalities during the preclinical stage of the disease, although careful auscultation when tidal volume of the foal is increased by exertion or use of a rebreathing bag can reveal localized wheezes and crackles or changes in the intensity of normal breath sounds (indicative of areas of lung consolidation).

Radiographic examination is a valuable aid in diagnosis and in monitoring progress in hospitalized foals.⁴³ Affected animals show evidence of consolidation of lung tissue, lymphadenopathy, and cavitating lesions in the lungs.

Ultrasonographic examination of the chest may reveal the presence of pulmonary consolidation before clinical signs are apparent and is useful means of detecting subclinical disease.

When lesions are confined to the intestinal wall the predominant clinical sign will be diarrhea, which may be acute or chronic and intermittent.

Intra-abdominal abscesses are associated with ill-thrift, weight loss, variable abdominal distension, fever, depression and in some cases colic. Ultrasonographic examination can reveal the abscess provided that it is within the field of the ultrasound probe.

PROGNOSIS

R. equi infection in Thoroughbred and Standardbred foals is associated with a reduced chance of racing as an adult compared with the overall population of foals, but affected foals that survive have a similar racing performance as adults to horses that did not have R. equi pneumonia.^22,44 The morbidity and case fatality rates are provide above under Epidemiology.

CLINICAL PATHOLOGY

Hematological evaluation usually reveals leukocytosis with neutrophilia and monocytosis, and elevation in the concentrations of acute phase proteins including plasma fibrinogen and serum amyloid A – changes characteristic, but not diagnostic, of R. equi infection.⁴⁵ Monitoring of blood white cell concentration and plasma fibrinogen concentration are useful in foals from farms on which the disease is endemic. White blood cell concentrations above 13.0 × 10⁹/L (13000 cells/μL) have a sensitivity and specificity of 95% and 61% respectively for R. equi pneumonia.⁴⁶ The high sensitivity means that few foals with the disease will be missed, while the moderate specificity means that a number of foals will be incorrectly suspected as having the disease. Because a high white cell count can be caused by a number of diseases other than R. equi pneumonia, foals with high white cell counts from farms on which the disease is endemic should be further examined for evidence of disease, including detailed clinical examination possibly including ultrasonographic examination, culture or PCR of tracheal aspirates, or thoracic radiography. Measurement of plasma fibrinogen concentration is less useful for detecting foals with R. equi pneumonia. Plasma fibrinogen concentrations of 400 mg/dL (0.4 g/L) have sensitivity and specificity of 91% and 51%, respectively, whereas concentrations of 600 mg/dL (0.6 g/L) have sensitivity and specificity of 38% and 96%, respectively.⁴⁶ The positive and negative predictive values of the tests depends on the prevalence of the disease among the group of foals examined, being low for farms on which the disease is rare and increasing as the prevalence of the disease increases. Serial measurement of serum amyloid A concentrations is not useful for detecting foals with clinically inapparent R. equi pneumonia, nor do foals with pneumonia reliably have higher serum amyloid A concentrations than normal foals.⁴⁷

Transtracheal aspirates from affected foals reveal a neutrophilic leukocytosis. Intracellular, Gram-positive pleomorphic rods characteristic of R. equi may be present in tracheal aspirates but the sensitivity of this observation has not been determined and all tracheal aspirates should be cultured.

Although numerous serological tests have been developed, including agar gel immunodiffusion, synergistic hemolysis inhibition, radial immunodiffusion and various ELISAs, none has demonstrated value in the diagnosis of the disease in individual animals. Currently available serological tests, either as single or paired samples, are not reliable in confirming or excluding the presence of R. equi pneumonia in foals.²⁸

Culture of tracheal aspirates is the gold standard for antemortem diagnosis of the disease, although sensitivity of culture is less than that of PCR examination of tracheal aspirates. Culture of tracheal aspirates has a sensitivity of approximately 86%, based on diagnosis of R. equi pneumonia at necropsy.⁴⁶ However, R. equi can be cultured from 35% of clinical normal foals in populations in which the disease is endemic.⁴⁸ A PCR test for the rapid detection of R. equi in tracheal aspirates has a sensitivity of 100% and a specificity of 91% in foals with a clinical diagnosis of R. equi pneumonia.⁴⁹ PCR examination of nasal swabs for presence of R. equi has a sensitivity of 15%, which is too low to be clinically useful.⁵⁰ More recent quantitative real-time PCR assays permit the rapid detection and quantification of virulent (VapA-gene-positive) strains of R. equi in tracheobronchial aspirates.⁵¹ This assay detects R. equi at concentrations as low as 20cfu/mL of tracheobronchial fluid, providing a specific and highly sensitive test for the presence of this organism. A multiplex PCR test simultaneously detects R. equi and the presence of virulence factors, thereby permitting rapid differentiation of pathogenic from nonpathogenic strains of R. equi in biological samples.⁵²

NECROPSY FINDINGS

The predominant lesions are a pyogranulomatous pneumonia plus lymphadenitis of the bronchial lymph nodes.¹² Grossly, the firm, raised lung nodules may reach several centimeters in diameter and be located anywhere in the lung field, especially in the cranioventral quadrant. If several nodules coalesce, the lesion may be misinterpreted as a suppurative bronchopneumonia. Histologically, organisms are easily demonstrated within the macrophages and giant cells comprising these lesions. Many cases also have ulcerative enterocolitis, with abscessation of mesenteric or cecocolic lymph nodes. Although necropsy may reveal widespread infection, many cases are subclinical.

DIAGNOSTIC CONFIRMATION

Antemortem diagnosis is by culture of R. equi from aspirates of tracheal fluid. Currently available serological tests do not provide confirmation of disease in individual animals.

DIFFERENTIAL DIAGNOSIS

The pneumonic form of the disease may be confused with other causes of pneumonia in foals (Table 16.4). Other causes of diarrhea in this age group include parasitism due to cyathostomes, infection by Salmonella sp. and antibiotic-induced diarrhea.

Table 16.4 Differential diagnosis of respiratory diseases of older (not newborn) foals

The aseptic synovitis and joint effusion that frequently accompanies R. equi pneumonia should be differentiated from septic arthritis due to S. zooepidemicus, Salmonella spp., R. equi or other bacteria.

TREATMENT

The principles of treatment are cure of R. equi infection, relief of respiratory distress and correction of associated immune-mediated diseases.

Elimination of infection requires the administration of antimicrobial agents that are both effective against the organism and able to penetrate infected macrophages to gain access to the organism. In vitro antibiotic sensitivity testing has not been demonstrated to be useful in predicting the clinical efficacy of treatment. R. equi isolates from ill foals are frequently sensitive in vitro to a variety of antibiotics, including the aminoglycosides gentamicin and neomycin, tetracycline, sulfonamides and chloramphenicol, while most are resistant to cephalosporins and penicillin.^53-55 However, treatment with antibiotics other than erythromycin and rifampin is associated with a lower recovery rate. Treatment with penicillin, with or without gentamicin, chloramphenicol or tetracycline, is not effective.⁵³ Trimethoprim–sulfadiazine combinations might be effective in some foals but are not the preferred treatment.⁵³ Neomycin has been recommended for treatment of R. equi pneumonia⁵⁶ but the risk of nephrotoxicosis, need for parenteral administration and lack of demonstration of clinical efficacy do not support its use at this time.

The treatment of R. equi pneumonia in foals is achieved by administration of macrolide antibiotics in combination with rifampin. Conventional treatment is administration of the combination of an acid-stable erythromycin (preferably estolate) at a dose of 25 mg/kg orally every 12 hours and rifampin at a dose of either 5 mg/kg every 12 hours or 10 mg/kg every 24 hours. Other esters or preparations of erythromycin are less well absorbed or have shorter elimination half-lives than the estolate ester and must be administered more frequently.³² Erythromycin ethylsuccinate does not provide optimal therapy for R. equi pneumonia in foals because of poor absorption after oral administration.^55,57 The macrolide antibiotics azithromycin and clarithromycin have also been used to treat foals with R. equi pneumonia. Treatment of foals with a combination of clarithromycin (7.5 mg/kg orally every 12 h) and rifampin results in improved survival over foals treated with azithromycin (10 mg/kg orally every 24 h) and rifampin or erythromycin and rifampin in a veterinary teaching hospital.⁵⁸

Therapy should be continued until the foal is clinically normal and has a normal plasma fibrinogen concentration and white blood cell count, which can require treatment for at least 1 month and often longer. Radiographic or ultrasonographic demonstration of resolution of the pulmonary consolidation and abscessation is useful in the decision to stop therapy. The case fatality rate is approximately 30% (see Epidemiology, above) even with appropriate treatment.

Side-effects of erythromycin–rifampin therapy include the development of diarrhea in some foals and their dams.⁵⁹ Administration of erythromycin to foals is associated with an eightfold increase in the risk of diarrhea.⁵⁰ Antibiotic therapy should be temporarily discontinued in foals that develop diarrhea.

During hot weather, some foals treated with erythromycin become hyperthermic (40–41°C, 104–105.5°F) and tachypneic and occasional deaths result from this syndrome.^50,60 The basis for this hyperthermic event, which may occur in healthy foals administered erythromycin, is unknown. Affected foals should be treated urgently with antipyretics, cold water bathing and housing in a cooler environment.

The emergence of R. equi isolates resistant to rifampin during therapy of foals with R. equi pneumonia has been reported.⁶¹ However, one case had an atypical clinical course, in that the foal was 10 months of age at presentation and may have been immunosuppressed, and in the other instance the foal was treated with only rifampin. The development of resistance during monotherapy with rifampin is a recognized contraindication to the use of this drug alone.

Ancillary therapy with NSAIDs, bronchodilators, and mucolytics might be of value.⁵³ Foals in severe respiratory distress require intranasal or intratracheal administration of oxygen.

CONTROL

Control measures are designed to maximize the resistance of the foal to infection and to reduce the infection pressure on the foal by decreasing contamination of the foal’s environment with virulent R. equi. Insuring adequate transfer of colostral immunoglobulins in all foals through routine monitoring of serum immunoglobulin concentrations in 1-day-old foals is an essential part of any control program. To decrease environmental contamination with virulent R. equi, efforts should be made to reduce fecal contamination of pastures and to reduce or eliminate dusty or sandy areas.^24,62 These efforts should include grassing or paving of bare areas, removal and composting of fecal material on a regular basis, reduction of stocking density and reduction in the size of mare/foal bands.²⁴

On farms with endemic disease, regular physical examination, including auscultation of foals and once-daily monitoring of rectal temperature, can permit early identification and treatment of affected foals.⁶³ Measurement of blood white cell count, as detailed above, can be useful in early identification of affected foals. Identification of one foal affected with R. equi pneumonia on a farm should prompt an examination of all other foals on the farm.

Ultrasonographic examination of the thorax of foals may permit identification of foals with clinically inapparent pulmonary abscesses.

The administration to foals of a hyperimmune serum, obtained from mares vaccinated with an autogenous vaccine, limits the severity of disease produced by experimental challenge but has not been consistently useful in preventing or decreasing the prevalence of naturally occurring disease.^19,34,64,65 There are no vaccines effective in prevention of R. equi pneumonia in foals.

ETIOLOGY

L. monocytogenes is widespread in nature and has characteristics that allow its survival and growth in a wide variety of environments. There is a highly diverse range of strains some of which have the capability of causing disease in animals and humans.

Optimal growth temperatures are between 30°C and 37°C but the organism can grow and reproduce at temperatures between –0.4°C and 45°C. It can grow between pH 4.5 and 9.6 although growth at low pH is minimal at low temperatures.¹ The organism is susceptible to common disinfectants.

L. monocytogenes can be divided into 16 serovars on the basis of somatic and flagellar antigens and there is considerable genetic diversity between serovars. Serovars 4b, ½a and ½b and 3 are most commonly isolated from diseased animals but there are geographical differences.^1-3 Virulent strains can multiply in macrophages and monocytes and produce a hemolysin, listeriolysin O, which is believed to be a major virulence factor.³

EPIDEMIOLOGY

PATHOGENESIS

In most animals, ingestion of the organism, with penetration of the mucosa of the intestine, leads to an inapparent infection with prolonged fecal excretion of the organism and to a subclinical bacteremia, which clears with the development of immunity. The bacteremic infection is frequently subclinical and may be accompanied by excretion of the organism in milk. Septicemic listeriosis, with or without meningitis, occurs most commonly in neonatal ruminants and in adult sheep and goats, particularly if they are pregnant and when the infection challenge is large.

The organism is a facultative intracellular pathogen that can infect cells, including intestinal cells, by directed endocytosis. It can survive and grow in macrophages and monocytes.³ Bacterial superoxide dismutase protects against the bactericidal activity of the respiratory burst of the phagocyte and listeriolysin O disrupts lysosomal membranes, allowing the organism to grow in the cytoplasm.^1,3 The experimental mouse model indicates that cell-mediated immunity is important in protection against listerial infection but studies in goats suggest that the clearance of bacteremic infection and resistance to infection are also strongly associated with humoral antibody.⁴⁴

In pregnant animals invasion of the placenta and fetus may occur within 24 hours of the onset of bacteremia. Edema and necrosis of the placenta leads to abortion, usually 5–10 days postinfection. Infection late in pregnancy results in stillbirths or the delivery of young that rapidly develop a fatal septicemia. Maternal metritis is constant and if the fetus is retained a fatal listerial septicemia may follow. Infection of the uterus causing abortion and intrauterine infection occurs in all mammals.

CLINICAL FINDINGS

When disease occurs it is usual to have an outbreak of either encephalitis or abortion. Encephalitis is the most prevalent manifestation in sheep. Septicemia in lambs may occur in conjunction with abortion but it is rare to have all three syndromes on the same farm, at least in the same temporal period. There are always exceptions to such generalities, and the occurrence of septicemia, abortion, and encephalitis in a flock of sheep is possible.²⁷

CLINICAL PATHOLOGY

The organism can be cultivated from vaginal secretions for up to 2 weeks after abortion and a proportion of aborting animals also have L. monocytogenes in the milk and feces.

The cerebrospinal fluid in cases of encephalitis has increased protein and an increased number of leukocytes, most of which are mononuclear cells or lymphocytes.^45,49 L. monocytogenes is not detectable by culture or PCR.⁵⁰ Electrodiagnostic examinations are reported.⁵¹

Serological tests (agglutination and complement fixation tests) have been used but lack the predictive value required for diagnostic use. Ruminants commonly have antibody to Listeria and high titers are commonly encountered in normal animals in flocks and herds where there have been clinical cases. Nucleic-acid-based techniques can be used to determine the source of a strain of L. monocytogenes in an outbreak.¹³

NECROPSY FINDINGS

Typically there are no distinctive gross changes associated with listerial encephalitis. Histological examination of central nervous system tissue is necessary to demonstrate the microabscesses that are characteristic of the disease. These are present in the brainstem in listerial encephalitis and in the cervical and/or lumbar spinal cord in outbreaks of spinal myelitis. Sampling of the forebrain will typically result in a false-negative diagnosis. Cold enrichment techniques are advisable when attempting to isolate the organism. Gram staining of paraffin-embedded tissue may permit confirmation of the diagnosis in cases for which suitable culture material is unavailable. Alternative test methods such as fluorescent antibody or immunoperoxidase tests are available in some laboratories. In one retrospective study comparing diagnostic methods, immunoperoxidase staining was superior to bacterial culture when correlated with histopathological changes.⁵²

Visceral lesions occur as multiple foci of necrosis in the liver, spleen, and myocardium in the septicemic form and in aborted fetuses. Aborted fetuses are usually edematous and autolyzed, with very large numbers of bacteria visible microscopically in a variety of tissues. In aborting dams, there is placentitis and endometritis in addition to the lesions in the fetus.³⁰

Sheep with enteritis show ulcerative abomasitis and some also have typhlocolitis at necropsy; histologically there are microabscesses throughout the intestine and a characteristic infiltration of degenerating neutrophils in the mucosa lamina muscularis of the abomasum.⁸

TREATMENT

The intravenous injection of chlortetracycline (10 mg/kg BW per day for 5 d) is reasonably effective in meningoencephalitis of cattle but less so in sheep. Penicillin at a dosage of 44000 IU/kg BW given intramuscularly daily for 7 days, and in many cases for 10–14 days, can also be used. The recovery rate depends largely on the time that treatment is started after the onset of clinical signs. If severe clinical signs are already evident, death usually follows in spite of treatment. Usually the course of events in an outbreak is that the first case dies but subsequent cases are detected sufficiently early for treatment. Dehydration, acid–base imbalances and electrolyte disturbances must also be corrected. Cases of spinal myelitis are poorly responsive to treatment.

Treatment of listerial iritis is with systemic antibiotics in the early stages coupled with subpalpebral corticosteroid and atropine to dilate the pupil.

In vitro resistance to the tetracycline group of antimicrobials is reported.⁵³

CONTROL

Control is difficult because of the ubiquitous occurrence of the organism, the lack of a simple method of determining when it is present in high numbers in the environment and a poor understanding of the risk factors other than silage. Where the risk factor is silage there may be some merit in the recommendation that a change of diet to include heavy feeding of silage should be made slowly, particularly if the silage is spoiled or if listeriosis has occurred on the premises previously. Tetracyclines can be fed in the ration of animals at risk in a feedlot. When possible, the obviously spoiled areas of silage should be separated and not fed.

Other recommendations on the feeding of silage include: avoiding making silage from fields where molehills may have contaminated the grass; avoiding soil contamination when filling the clamp; the use of additives to improve fermentation and the avoidance of silage that is obviously decayed, or with a pH of greater than 5 or an ash content of more than 70 mg/kg of dry matter.

Silage removed from the clamp should be fed as soon as possible.

Where uveitis is a problem, feeding systems that avoid eye contact with silage should be used.

A live attenuated vaccine has been shown to induce protection against intravenous challenge⁵⁴ and a live attenuated vaccine in use in Norway for several years is reported to reduce the annual incidence of the disease in sheep from 4% to 1.5%.⁵⁵ An economic model is available for determining whether vaccination should be practiced.⁵⁶ Commercial killed vaccines are available for the control of the disease in some countries and some companies will also produce autogenous vaccines on request. The efficacy of vaccination still requires further determination; however; when economics or food availability on the farm dictate that contaminated silage must be fed, consideration might be given to vaccination as a means of providing some protection.

ETIOLOGY

Erysipelothrix rhusiopathiae (insidiosa) is the causative bacterium and the disease can be produced in acute and subacute septicemic and chronic forms by the injection of cultures of the organism. A number of different serotypes¹ have been identified, usually types 1 and 2 in pigs. Many of the serotypes have been regrouped and called Erysipelothrix tonsillarum. This is a nonpathogenic type found in the tonsil that is morphologically and biochemically similar to E. rhusiopathiae but has a very distinctive genetic profile. In many minimal-disease herds they have tended not to vaccinate and then the epizootics have occurred as a result of an increasing lack of immunity.²

EPIDEMIOLOGY

PATHOGENESIS

The invasion of the susceptible pig by E. rhusiopathiae can occur under particular circumstances, i.e. if weather conditions are hot and humid or in particular fields or buildings. Experimentally, it is often easier to infect the pig through scarified wounds than through intravenous infusions.

No specific virulence factors have been found but there is the presence in the pathogenic serotypes of an ability to produce a capsule that resists phagocytosis.²⁷ Some others may produce a neuraminidase, which may cleave the mucopolysaccharides in cell walls and cause vascular damage leading to hemorrhage and thrombosis. Invasion of the bloodstream occurs in all infected animals in the first instance. Septicemia results within 1–7 days. The subsequent development of either an acute septicemia or a bacteremia with localization in organs and joints is dependent on undetermined factors. Virulence of the particular strain may be important and this may depend upon the number of recent pig passages experienced. Coagulase activity is a possible virulence factor.²⁸ Concurrent viral infection, especially hog cholera, may increase susceptibility of the host.

Localization in the chronic form is commonly in the skin and joints, and on other heart valves, with probable subsequent bacteremic episodes, and it may start from as early as 4 days after initial infection, although the cartilage lesions may be delayed until about 8 months and they can then continue to progress for at least 2 years. Selective adherence of some strains of E. rhusiopathiae (insidiosa) to heart valves may be a factor in the pathogenesis of endocarditis.²⁹ In joints, the initial lesion is an increase in synovial fluid and hyperemia of the synovial membrane, followed in several weeks by the proliferation of synovial villi (really a synovitis), thickening of the joint capsule and enlargement of the local lymph nodes. Diskospondylitis also occurs in association with chronic polyarthritis due to erysipelas.³⁰ Amyloidosis may occur in pigs with chronic erysipelas polyarthritis.³¹ The heart lesions may begin with early inflammatory changes associated with emboli.

There has been some controversy over whether the arthrodial lesions result from primary infection or whether they result from hypersensitivity to the Erysipelothrix or other antigen. Current opinion suggests that the former is the case but that the lesions are enhanced by immunological mechanisms to persistent antigen at the site. There are increased levels of immunoglobulins IgG and IgM in the synovial fluids of pigs with polyarthritis due to E. rhusiopathiae (insidiosa)³² and the levels are considered only partly due to serum and increased permeability.

Experimentally the disease can be produced by oral dosing, by intradermal, intravenous, and intra-articular injection, and by application to scarified skin, conjunctiva, and nasal mucosa. The arthritic form of the disease can be reproduced by multiple intravenous inoculations of E. rhusiopathiae.¹

The microscopic lesions include vasculitis in capillaries and venules in many sites, including glomeruli, pulmonary capillaries, and the skin. Sometimes, it is possible to see emboli of bacteria without specific stains to demonstrate bacteria.

CLINICAL FINDINGS

CLINICAL FINDINGS

NECROPSY FINDINGS

DIAGNOSIS

Clinical signs, isolation of the agent from multiple sites including blood in the acute stages; but diagnosis from joints in chronic stages is more difficult.

TREATMENT

CONTROL

Successful control depends on good hygiene, biosecurity (other pigs and other species), reduction of stress, an effective 6-monthly vaccination policy, preferably two doses, for all animals including boars over 3 months of age, as well as rapid diagnosis, quarantine, and treatment.

ETIOLOGY

Bacillus anthracis is the specific cause of the disease, and pathogenic strains have plasmid-encoded virulent factors: a poly-D-glutamic capsule, which aids in resistance to phagocytosis and is encoded by virulence plasmid pX02, and a tripartite toxin comprised of edema (factor I) lethal (factor II) and protective antigen (factor III) encoded by plasmid pX01.^1,2 Both plasmids are required for full virulence and avirulent strains occur.³ There is little diversity among isolates of B. anthracis but genotyping can be used for epidemiological studies.⁴ The organism forms spores that persist in the environment for decades.

EPIDEMIOLOGY

PATHOGENESIS

Upon ingestion of the spores, infection may occur through the intact mucous membrane, through defects in the epithelium around erupting teeth, or through scratches from tough, fibrous food materials. The organisms are resistant to phagocytosis, in part due to the presence of the poly-d-glutamic acid capsule, and proliferate in regional draining lymph nodes, subsequently passing via the lymphatic vessels into the bloodstream; septicemia, with massive invasion of all body tissues, follows. B. anthracis produces a lethal toxin that causes edema and tissue damage, death resulting from shock and acute renal failure and terminal anoxia.

In pigs, localization occurs in the lymph nodes of the throat after invasion through the upper part of the digestive tract. Local lesions usually eventually lead to a fatal septicemia.

CLINICAL FINDINGS

The incubation period after field infection is not easy to determine but is probably 1–2 weeks.

CLINICAL PATHOLOGY

Hematology and blood chemistry examinations are not conducted because of the risk for human exposure. In the living animal the organism may be detected in a stained smear of peripheral blood. The reference standard for diagnosis is the detection, by microscopic examination, of a clearly defined metachromatic capsule on square-ended bacilli (often in chains) in a blood smear stained with aged polychrome methylene blue. The blood should be carefully collected in a syringe to avoid contamination of the environment. When local edema is evident smears may be made from aspirated edema fluid or from lymph nodes that drain that area. For a more certain diagnosis, especially in the early stages when bacilli may not be present in the bloodstream in great numbers, blood culture or the injection of syringe-collected blood into guinea-pigs is satisfactory.

Fluorescent antibody techniques are available for use on blood smears and tissue sections. Monoclonal antibodies are also used to provide specific identification of anthrax organisms.¹⁶

As the carcass decomposes and the vegetative forms of B. anthracis die, diagnosis by smear is more difficult and an immunochromatographic test for protective antigen has been developed that has high specificity and does not give positive results in recently vaccinated cattle.¹⁷ In cases where antibiotic therapy has been used, the identification from blood smears or culture may be difficult and animal passage may be necessary. Isolation of anthrax bacilli from infected soil is difficult, but a real-time quantitative PCR with reported high sensitivity has been described.¹⁸

NECROPSY FINDINGS

There is a striking absence of rigor mortis and the carcass undergoes gaseous decomposition, quickly assuming the characteristic ‘sawhorse’ posture. All natural orifices usually exude dark, tarry blood that does not clot. If there is a good reason to suspect the existence of anthrax the carcass should not be opened. If a necropsy is carried out, the failure of the blood to clot, widespread ecchymoses, bloodstained serous fluid in the body cavities, severe enteritis and splenomegaly are strong indications of the presence of anthrax. The enlarged spleen is soft, with a consistency likened to ‘blackberry jam’. Subcutaneous swellings containing gelatinous material, and enlargement of the local lymph nodes are features of the disease in horses and pigs. Lesions are most frequently seen in the soft tissues of the neck and pharynx in these species.

To confirm the diagnosis on an unopened carcass, peripheral blood or local edema fluid should be collected by needle puncture. Since the blood clots poorly, jugular venipuncture may permit sample collection. Smears prepared from these fluids should be stained with polychrome methylene blue and examined (see Clinical pathology, above). These fluid samples can also be used for bacteriological culture if smear results are equivocal. The smears should be prepared and interpreted by an experienced and qualified microbiologist.

If decomposition of a carcass is advanced, a small quantity of blood may be collected from the fresh surface of an amputated tail or ear. A portion of spleen is the specimen of choice for bacteriological culture if the carcass has been opened. An immunofluorescence test is available but cross-reactivity with other Bacillus spp. makes its use impractical. An immunochromatographic test that has high specificity for protective antigen has been developed for use in decomposed carcasses.¹⁷

The Ascoli test can be used to demonstrate antigen in severely decayed tissue samples and a nested PCR technique has been used to demonstrate antigen in environmental samples;¹⁹ PCR methods can also be used to confirm the identity of bacterial isolates. If other detection methods fail, experimental animal inoculation can be attempted. Immunohistochemical detection of the bacilli in skin biopsies of cutaneous anthrax in humans has been described.²⁰ This technique may be useful in retrospective analyses of suspect cases if suitable fresh tissue samples were not collected.

Anthrax is a reportable disease in many countries, requiring the involvement of government regulatory agencies when the disease is suspected or when the diagnosis is confirmed. Representatives of these agencies can often facilitate sample collection and transportation to an appropriate laboratory. If anthrax is suspected, then shipping diagnostic samples via the mail or courier systems is strongly discouraged. Instead, samples should be appropriately packed, labeled and transported directly to the laboratory by one of the staff members of the veterinary clinic. In some countries it may be illegal to send material such as anthrax through the mail system.

TREATMENT

Severely ill animals are unlikely to recover but in the early stages, particularly when fever is detected before other signs are evident, recovery can be anticipated if the correct treatment is provided. Penicillin (20000 IU/kg BW twice daily) has had considerable vogue, but streptomycin (8–10 g/d in two doses intramuscularly for cattle) is much more effective. Oxytetracycline (5 mg/kg BW per day) parenterally has also proved superior to penicillin in the treatment of clinical cases after vaccination in cattle and sheep. Antiserum, if available, should also be administered for at least 5 days in doses of 100–250 mL daily but it is expensive.

In vitro studies show all isolates to be susceptible to ampicillin, streptomycin, erythromycin, tetracycline, methicillin, and netilmicin.²¹ It is desirable to prolong treatment to at least 5 days to avoid a recrudescence of the disease.

CONTROL

The control of meat- and milk-producing animals in infected herds in such a way as to avoid any risk to the human population is a special aspect of the control of anthrax. It is necessary at the same time to avoid unnecessary waste and the imposition of unnecessarily harsh prohibitions on the farmer. When an outbreak occurs, the placing of the farm in quarantine, the destruction of discharges and cadavers, and the vaccination of survivors, are part of the animal disease control program and indirectly reduce human exposure. Prohibition of movement of milk and meat from the farm during the quarantine period should prevent entry of the infection into the human food chain. Vaccination of animals, although the vaccine is a live one, does not present a hazard to humans, although there is a withholding period for meat and milk after its use.

Disposal of infected material is most important and hygiene is the biggest single factor in the prevention of spread of the disease. Infected carcasses should not be opened but immediately burned in situ or buried, together with bedding and soil contaminated by discharges. If this can not be done immediately, a liberal application of 5% formaldehyde on the carcass and its immediate surroundings will discourage scavengers. Burning is the preferred method of disposal. Approximately one cord of wood is required to effectively incinerate the carcass of a mature cow. Bags of charcoal briquettes have also been used.⁵

Burial should be at least 2 m deep with an ample supply of quicklime added. All suspected cases and in-contact animals must be segregated until cases cease and for 2 weeks thereafter the affected farm must be kept in quarantine to prevent the movement of livestock. The administration of hyperimmune serum to in-contact animals may prevent further losses during the quarantine period, but prophylactic administration of a single dose of long-acting tetracycline or penicillin is a much commoner tactic.

Disinfection of premises, hides, bone meal, fertilizer, wool, and hair requires special care. When disinfection can be carried out immediately, before spore formation can occur, ordinary disinfectants or heat (60°C (140°F) for a few minutes) are sufficient to kill vegetative forms. This is satisfactory when the necropsy room or abattoir floor is contaminated. When spore formation occurs (i.e. within a few hours of exposure to the air), disinfection is almost impossible by ordinary means. Strong disinfectants such as 5% Lysol require to be in contact with spores for at least 2 days. Strong solutions of formalin or sodium hydroxide (5–10%) are probably most effective. Peracetic acid (3% solution) is an effective sporicide and, if applied to the soil in appropriate amounts (8 L/m²), is an effective sterilant. Infected clothing should be sterilized by soaking in 10% formaldehyde. Shoes may present a difficulty and sterilization is most efficiently achieved by placing them in a plastic bag and introducing ethylene oxide. Contaminated materials should be damp and left in contact with the gas for 18 hours. Hides, wool, and mohair are sterilized commercially by gamma-irradiation, usually from a radioactive cobalt source. Special care must be taken to avoid human contact with infected material and, if such contact does occur, the contaminated skin must be thoroughly disinfected. The source of the infection must be traced and steps taken to prevent further spread of the disease. Control of the disease in a feral animal population presents major problems.